There is a word count on Hubs, but it does not show up at first. On Hubs that have been published, and on Hubs that have not been published but that you have "done editing," you can click the Stats button at the top of the Hub (when you are signed-in to your account). Click the "Hub Metrics" tab, and the first line is the Word Count.
When you first work on a Hub, that number will not show up, though; it takes a little time for the system to check it and show the number (I believe... I'm pretty sure... :) ). If you are wanting to keep up with a word count on Hubs-in-progress, the best thing to do is either to compose in a word-processing program (as mentioned in another answer) or to estimate as you go.
You can develop an eye for estimating a word count by doing this: Look at some of the Hubs you have already published and see how much of the screen the words fill up. How many times do you need to scroll down to finish seeing the words? Check the word count. Use those clues (known word count; appearance on-screen; number of scroll-downs) to help with estimating - but remember that it is an estimate only, and the actual word count will show up in the stats after the system finishes checking.